1. Technical Field
The present invention relates to a reception signal integrating method and a receiver.
2. Related Art
As a positioning system that utilizes a positioning signal, a GPS (Global Positioning System) is widely known. The GPS is used in a GPS receiver incorporated in a cellular phone, a car navigation system, and the like. The GPS receiver performs location calculation processing for calculating, on the basis of information such as the locations of plural GPS satellites and pseudo distances from the GPS satellites to the receiver, three-dimensional coordinate values indicating the location of the receiver and a clock error.
A GPS satellite signal is a type of a communication signal spread and modulated in a CDMA (Code Division Multiple Access) system conventionally known as a spread spectrum modulation system. As a method generally used in acquiring the GPS satellite signal from a reception signal, there is a method of performing correlation processing for the reception signal and a replica signal of a CA (Coarse and Acquisition) code, which is a spread code of the GPS satellite signal, while changing a frequency and a code phase (so-called correlation operation of a frequency direction and a phase direction; also referred to as frequency search and phase search) to determine a frequency and a code phase (e.g., JP-A-2007-256111).
In an environment in which the reception signal of the GPS satellite signal is in a weak electric field (e.g., an indoor environment; hereinafter referred to as “weak electric field environment”), since the reception signal is feeble, a difference does not appear in correlation values obtained by performing the correlation processing and, in some case, a peak of the correlation values is unknown. Therefore, a method of facilitating discrimination of the peak of the correlation values by performing, in the weak electric field environment or the like, integrating the reception signal over a predetermined period and performing the correlation processing for the integrated signal is used.
A carrier frequency of the GPS satellite signal is 1.57542 [GHz]. The CA code, which is the spread code of the GPS satellite signal, is a pseudo random noise code having a repetition period of 1 ms with a code-length 1023 chip set as a 1PN frame. A chip rate of the CA code is 1.023 [MHz]. Therefore, theoretically, the number of periods of a carrier wave per one chip is 1540. The number of periods per one code period of the CA code is 1540×1023=1,575,420.
However, a reception frequency in actual reception of the GPS satellite signal includes frequency errors due to a so-called Doppler frequency and an error of a local clock (a clock error). Because of the presence of these frequency errors, a code period estimated on a receiver side as one period of the CA code that should originally be a repetition period of 1 ms (this period is hereinafter referred to as “assumed period”) deviates from a true period. At every 1,575,420 periods of the carrier wave, a period of the CA code is one code period. This period is the “true period”. However, the receiver does not count a period of the carrier wave to determine a code period. The receiver assumes and determines the code period without counting the period of the carrier wave. More specifically, the receiver determines a frequency to equivalently assume the code period without directly assuming the code period.
Therefore, when the reception signal is time-divided, the carrier wave is not always time-divided at exactly the 1,575,420 periods and a some shift of a phase occurs. In other words, since the carrier wave is time-divided by assuming the code period, a shift occurs between a phase of the carrier wave at the start point of a certain code period and a phase of the carrier wave at the start point of the next code period (this shift is hereinafter referred to as “period shift”). The period shift is equivalent to a shift between the true period and the assumed period of the CA code. As explained above, the assumption of the code period can be synonymous with calculation of a reception frequency. Since the frequency errors are included in the reception frequency, the period shift occurs.
Even if it is attempted to integrate the reception signal in order to facilitate discrimination of a peak of the correlation values in the weak electric field environment, when the reception signal is integrated in a state in which the period shift occurs, in some case, the amplitude of the reception signal decreases on the contrary. This is because, when a signal is integrated while a period cannot be accurately grasped, a phase of the signal to be integrated shifts and it is likely that the signal with plus and minus of amplitude reversed is integrated. When the reception signal is integrated and the correlation processing is performed in a state in which the period shift occurs, it is also likely that a discriminated peak of correlation values is not a correct result.